In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment may cause problems and should be prevented. Further, in some climates, the development of ice within an optical fiber cable may have a crushing influence on the optical fibers in the core which may affect adversely the attenuation thereof.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and into the core. For example, a metallic shield which often times is used to protect a cable against electromagnetic interference is provided with a sealed longitudinal seam. However, because lightning strikes may cause holes in the metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Filling materials have been and are being used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. However, the application of a filling material to a cable core and to sheath components during cable manufacturing presents some housekeeping problems and inhibits somewhat line speeds because of the need to fill carefully interstices of the core.
Presently, many commercially available cables also include a water swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two cellulosic tissues. During manufacture of the laminated tape, the powder is exposed to a mist to cause the portions of the laminate to remain together. Further included may be a polyester scrim which is used to provide tensile strength for the laminated tape. Such a tape which provides suitable water protection for the cable must not be too thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
A problem which has surfaced as the result of using water blocking tapes relates to microbic growth. The use of tissue-based tapes for water blocking purposes may lead to the growth of fungus. Microbic growth is not desired because it may affect adversely polymeric materials such as by the removal of plasticizers, modifiers and lubricants which result in changes in physical properties and deterioration of electrical properties, for example. Desirably, cables should be free of microbic growth and hence of materials which may engender such growth.
What is needed and what does not appear to be available in the marketplace is a microbial resistant water blocking member, preferably tape-like, which is relatively thin and relatively inexpensive. Such a water blocking member should be one which is compressible and which has acceptable tensile properties. Because in some optical fiber cables, the water blocking member is engaged by helically wound metallic strength members, it should be able to conform to the configurations of those members and to allow those members to become embedded therein without destroying the water blocking effectiveness. If the water blocking member has this capability, commonly used strength member wires will not move about and will provide torsional stability from layer to layer. On the other hand, if the water blocking member does not have this capability and if all the wires were to assume positions on one portion of the periphery, the cable would not be balanced torsionally and would be very difficult to bend.
Care also must be taken to avoid problems caused by what is referred to as bleed-through of molten plastic jacketing material. With a water blocking member comprised of a highly porous substrate the greater the line speed the greater the flow of the molten plastic material into the substrate and the more difficult it becomes to strip the jacket to expose the core. As a result, the use of a highly porous water blocking member may severely limit the line speed.
What is needed and what seemingly is not available in the prior art is a cable which includes a microbial resistant water blocking member. The sought after member should be one which is relatively low in cost, which is easily incorporated into the cable design and which is easily handled. Also, it should be compressible and should have a suitable porosity.